The FTSE 100 Index, also called FTSE 100, FTSE, or, informally, the 'footsie' ( /ˈfʊtsiː/), is a share index of the stocks of the 100 companies listed on the London Stock Exchange having the highest market capitalisation.

The index is maintained by the FTSE Group, an independent company joint owned by the Financial Times and the London Stock Exchange. Its name derives from the acronym of its two parent companies, but has since been registered as a limited company in its own right.

The index began on 4 January 1984 with a base level of 1000; the highest value reached to date is 6950.6, on 30 December 1999. After falling dramatically during the financial crisis of 2007-2010 to a low of below 3,500 in March 2009, the index recovered by a significant margin to its peak since then of 6,091.33 on 8 February 2011, its highest level since mid-2008. It then fell sharply to under the 5,000 mark on the morning of 23 September 2011. As of 19 April 2012 it was at 5,744.

It is the most widely used of the FTSE Group's indices, and is frequently reported (e.g. on UK news bulletins) as a measure of business prosperity.